The present invention relates to cellulose materials, i.e. to cellulose or to cellulose derivatives, to liquid-crystal solutions based on such cellulose materials, in particular to spinnable solutions capable of yielding, after coagulation, spun articles such as fibres or films, to these spun articles themselves, and also to processes for obtaining such spun articles.
The invention relates more particularly to an aqueous coagulating agent suitable for coagulating liquid-crystal solutions based on cellulose materials, the use of such a coagulating agent for coagulating such solutions, in particular in a spinning process, and also to a novel cellulose fibre having an unexpected combination of mechanical properties.
It has been known for a long time that the production of liquid-crystal solutions is essential for obtaining fibres having high or very high mechanical properties by spinning, as has been shown in particular by Patents U.S. Pat. No. 3,767,756, which relates to aramid fibres, and U.S. Pat. No. 4,746,694, which relates to aromatic polyester fibres. The spinning of liquid-crystal solutions of cellulose also makes it possible to obtain fibres having high mechanical properties, in particular by what is called the xe2x80x9cdry-jet-wet spinningxe2x80x9d processes, as described, for example, in International Patent Applications PCT/CH85/00065 and PCT/CH95/00206 for liquid-crystal solutions based on cellulose and at least one phosphoric acid.
Patent Application PCT/CH85/00065, published under No. WO85/05115, or its equivalent patents EP-B-179 822 and U.S. Pat. No. 4,839,113, describe the obtaining of spinning solutions based on cellulose formate, by reacting the cellulose with formic acid and phosphoric acid, these solutions being in the liquid-crystal state. These documents also describe the spinning of these solutions using what is called the xe2x80x9cdry-jet-wet spinningxe2x80x9d technique to obtain cellulose formate fibres, as well as cellulose fibres regenerated from these formate fibres.
Patent application PCT/CH95/00206, published under No. WO96/09356, describes a method for dissolving cellulose directly, without formic acid, in a solvent in order to obtain a liquid-crystal solution, this solvent containing more than 85% by weight of at least one phosphoric acid. The fibres obtained after spinning this solution are fibres of non-regenerated cellulose.
Compared with conventional cellulose fibres such as rayon or viscose fibres, or with other conventional non-cellulose fibres, such as nylon or polyester fibres, for example, all spun from optically isotropic liquids, the cellulose fibres described in these two applications WO85/05115 and WO96/09356 are characterised by a far more ordered or oriented structure, owing to the liquid-crystal nature of the spinning solutions from which they have originated. They have very high mechanical properties in extension, in particular toughnesses of the order of 80 to 120 cN/tex, or even more, and initial moduli which may exceed 2500 to 3000 cN/tex.
However, the processes described in the above two applications for obtaining these fibres having very high mechanical properties all have the same disadvantage: the coagulation step is performed in acetone.
Now, acetone is a relatively costly, volatile product, which furthermore has a risk of explosion which requires special safety measures. Such disadvantages are furthermore not peculiar to acetone, but in fact common to numerous organic solvents used in the spinning industry, in particular as coagulating agents.
It was therefore entirely desirable to find an alternative to the use of acetone by replacing it with a coagulating agent which would be more advantageous from an industrial point of view and easier to use, even at the expense of a reduction of certain mechanical properties of the fibres obtained, particularly since the very high mechanical properties described above may be excessive for certain technical applications.
Although it has proved technically possible to replace the acetone with water to coagulate the liquid-crystal solutions described in the two applications WO85/05115 and WO96/09356 mentioned above, experience has shown that the use of water instead of acetone resulted in spinning difficulties and in cellulose fibres having very low toughness compared with those described above, this toughness scarcely ever exceeding 30-35 cN/tex, and reaching at most only 35-40 cN/tex when the fibre being formed is subjected, for example, to particularly high tensile stresses, which furthermore are detrimental to the quality of the product obtained. Such values of 30 to 40 cN/tex are in any case lower than the known toughness values of a conventional fibre of the rayon type (40-50 cN/tex), which nevertheless is obtained from a non-liquid-crystal spinning solution, i.e. one which is optically isotropic.
Thus, for spinning liquid-crystal solutions based on cellulose materials, water has proved to be a coagulating agent which is incapable of producing fibres having satisfactory mechanical properties, in particular a toughness at least equal to that of a conventional rayon fibre, for technical applications, for example for reinforcing rubber articles or tires.
A first aim of the present invention is to propose a novel, water-based coagulating agent which is more advantageous from the industrial point of view than acetone and more effective than water alone, which is capable of producing fibres, the toughness and modulus properties of which are substantially improved compared with those of fibres coagulated simply with water.
The aqueous coagulating agent according to the invention, which is capable of coagulating a liquid-crystal solution based on cellulose materials, is characterised in that it comprises at least one water-soluble additive, selected from the group consisting of ammonia, amines or the salts of these compounds, the additive being such that the pH of said coagulating agent is greater than 6.
The invention also relates to a process for spinning a liquid-crystal solution based on cellulose materials, for obtaining a spun article, effected using a coagulating agent according to the invention, and also to any spun article obtained by such a process.
Another aim of the invention is to propose a novel cellulose fibre which may be obtained by the process according to the invention; this novel fibre, compared with a conventional rayon fibre, has a toughness at least equal to, if not greater than, a comparable fatigue strength, all combined with a significantly higher initial tensile modulus.
The cellulose fibre of the invention has the following characteristics:
its toughness T is greater than 40 cN/tex;
its initial tensile modulus Im is greater than 1200 cN/tex;
its breaking load degeneration xcex94F after 350 fatigue cycles in what is called the xe2x80x9cbar testxe2x80x9d, at a compression ratio of 3.5% and a tensile stress of 0.25 cN/tex, is less than 30%.
The invention furthermore relates to the following products:
reinforcement assemblies comprising at least one spun article according to the invention, for example, cables, plied yarns, multifilament fibres twisted on themselves, such reinforcement assemblies possibly being, for example, hybrids, composites, i.e. comprising elements of different natures, possibly not in accordance with the invention;
articles reinforced by at least one spun article and/or an assembly according to the invention, these articles being, for example, articles made of rubber or of plastics material(s), for example plies, belts, tubes or tires, in particular tire carcass reinforcements.
The invention and its advantages will be readily understood in the light of the following description and non-limiting examples.
I-1. Degree of Substitution
The degree of substitution (DS) of the fibres regenerated from a cellulose derivative, for example from cellulose formate, is measured in known manner, as indicated hereafter: approximately 400 mg of fibre is cut into pieces of a length of 2-3 cm, then weighed out with precision and introduced into a 100 ml Erlenmeyer flask containing 50 ml of water. 1 ml of normal caustic soda solution (1N NaOH) is added. The mixture is mixed at ambient temperature for 15 minutes. The cellulose is thus completely regenerated by transforming the last substituent groups which had resisted the regeneration treatment on continuous fibres into hydroxyl groups. The excess sodium hydroxide is titrated with a decinormal solution of hydrochloric acid (0.1 N HCl), and the degree of substitution is thus deduced therefrom.
I-2. Optical Properties of the Solutions
The optical isotropy or anisotropy of the solutions is determined by placing a drop of test solution between the linear crossed polariser and analyser of an optical polarisation microscope, followed by observing this solution at rest, that is to say in the absence of dynamic stress, at ambient temperature.
In known manner, an optically anisotropic solution, also referred to as a liquid-crystal solution, is a solution which depolarises light, that is to say, which when thus placed between a linear crossed polariser and analyser transmits light (coloured texture). An optically isotropic solution, that is to say, one which is not a liquid-crystal solution, is a solution which, under the same observation conditions, does not have the above property of depolarisation, the field of the microscope remaining black.
I-3. Mechanical Properties of the Fibres
The term xe2x80x9cfibresxe2x80x9d is understood here to multi-filament fibres (also called spun yarns), consisting, in known manner, of a large number of elementary filaments of small diameter (low linear density). All the mechanical properties below are measured on fibres which have undergone prior conditioning. The term xe2x80x9cprior conditioningxe2x80x9d is understood to refer to the storage of the fibres, before measurement, in a standard atmosphere in accordance with European Standard DIN EN20139 (temperature of 20xc2x12xc2x0 C.; moisture content of 65xc2x12%) for at least 24 hours. For fibres of cellulose material, such prior conditioning makes it possible to stabilise their moisture content at an equilibrium level of less than 15% by weight of dry fibre.
The linear density of the fibres is determined on at least three samples, each corresponding to a length of 50 m, by weighing this length of fibre. The linear density is given in tex (weight in grammes of 1000 m of fibre).
The mechanical properties in extension (toughness, initial modulus and elongation at break) are measured in known manner using a Zwick GmbH and Co (Germany) 1435-type or 1445-type tension machine. After receiving a low prior protective twist (helical angle of about 6xc2x0), the fibres undergo tension over an initial length of 400 mm, at a nominal speed of 200 mm/min, or at a speed of 50 mm/min if their elongation at break does not exceed 5%. All the results given are an average of 10 measurements.
The toughness T (breaking load divided by linear density) and the initial tensile modulus, Im, are indicated in cN/tex (centinewtons per tex). The initial modulus Im is defined as the gradient of the linear part of the force-elongation curve, which occurs just after a standard pretension of 0.5 cN/tex. The elongation at break, referred to as Eb, is indicated as a percentage (%).
I-4. Resistance to the xe2x80x9cBar Testxe2x80x9d
A simple test, referred to as the xe2x80x9cbar testxe2x80x9d, is used to determine the fatigue strength of the fibres studied.
For this test, a short length of fibre (length at least 600 mm) which has been subjected to prior conditioning is used, the test being performed at ambient temperature (about 20xc2x0 C.). This length, subjected to a tension of 0.25 cN/tex due to a constant weight fixed to one of its free ends, is stretched over a bar of polished steel, and curved around the latter at an angle of curvature of about 90 degrees. A mechanical device to which the other end of the length of fibre is fixed ensures forced, repeated sliding of the fibre on the polished steel bar, in an alternating linear movement of given frequency (100 cycles per minute) and amplitude (30 mm). The vertical plane containing the axis of the fibre is always substantially perpendicular to the vertical plane containing the bar, which is itself horizontal.
The diameter of the bar is selected to cause a compression of 3.5% upon each pass of the filaments of the fibre around the bar. By way of example, a bar of a diameter of 360 xcexcm (micrometers) is used for a fibre having an average diameter of the filaments of 13 xcexcm (or an average linear density of the filaments of 0.20 tex, for a density of cellulose of 1.52).
The test is terminated after 350 cycles, and the breaking load degeneration after fatigue, referred to as xcex94F, is measured, in accordance with the equation:
xcex94F (%)=100[F0xe2x88x92F1]/F0
F0 being the breaking load of the fibre before fatigue, and F1 its breaking load after fatigue.
First of all, the conditions for preparing the liquid-crystal solutions based on cellulose materials will be described (xc2xa7II-1), then the conditions of spinning of these solutions to obtain fibres (xc2xa7II-2).
II-1. Preparation of the Solutions
The liquid-crystal solutions are prepared in known manner, by dissolving the cellulose materials in an appropriate solvent or solvent mixturexe2x80x94referred to as xe2x80x9cspinning solventxe2x80x9dxe2x80x94as indicated, for example, in applications WO85/05115 and WO96/09356 referred to above.
xe2x80x9cSolutionxe2x80x9d is understood here, in known manner, to mean a homogenous liquid composition in which no solid particle is visible to the naked eye. xe2x80x9cLiquid-crystal solutionxe2x80x9d is understood to mean a solution which is optically anisotropic at ambient temperature (about 20xc2x0 C.) and at rest, i.e. in the absence of any dynamic stress.
Preferably, the coagulating agent of the invention is used to coagulate liquid-crystal solutions containing at least one acid, this acid more preferably belonging to the group consisting of formic acid, acetic acid, phosphoric acids or mixtures of these acids.
The coagulating agent of the invention may advantageously be used to coagulate:
liquid-crystal solutions of cellulose derivatives based on at least one phosphoric acid, these solutions being in particular solutions of cellulose esters, in particular cellulose formate solutions, such as described, for example, in application WO85/05115 referred to above, produced by mixing cellulose, formic acid and phosphoric acid (or a liquid based on phosphoric acid), the formic acid being the esterification acid, the phosphoric acid being the solvent of the cellulose formate;
liquid-crystal solutions of cellulose based on at least one phosphoric acid, such as described for example in application WO96/09356 referred to above, prepared by directly dissolving the cellulose, i.e. without derivation, in a suitable solvent containing more than 85% by weight of at least one phosphoric acid complying with the following average formula:
[n(P2O5),p(H2O)], with: 0.33 less than (n/p) less than 1.0.
The starting cellulose may be in various known forms, in particular in the form of a powder, prepared for example by pulverising a cellulose plate in the raw state. Preferably, its initial water content is less than 10% by weight, and its DP (degree of polymerisation) is between 500 and 1000.
The appropriate mixing means for obtaining a solution are known to the person skilled in the art: they must be capable of correctly kneading and mixing, preferably at a controllable speed, the cellulose and the acids until the solution is obtained. The mixing can be carried out, for example, in a mixer comprising Z-shaped arms or in a mixer with a continuous screw. These mixing means are preferably equipped with a device for evacuation under vacuum and with a heating and cooling device which makes it possible to adjust the temperature of the mixer and its contents, in order to accelerate, for example, the dissolving operations, or to control the temperature of the solution during formation.
By way of example, for a cellulose formate solution, the following operating method can be used: an appropriate mixture of orthophosphoric acid (99% crystalline) and formic acid is introduced into a dual-casing mixer, comprising Z-shaped arms and an extrusion screw. Then powdered cellulose is added (the moisture content of which is in equilibrium with the ambient air humidity); the entire batch is mixed for a period of about 1 to 2 hours, for example, the temperature of the mixture being kept between 10 and 20xc2x0 C. until a solution is obtained. It is possible to proceed in the same manner for a solution in accordance with application WO96/09356, by replacing the formic acid, for example, with a polyphosphoric acid.
The solutions thus obtained are ready for spinning, and can be transferred directly, for example by means of an extruder screw placed at the mixer outlet, to a spinning machine in order to be spun thereon, without any prior transformation other than usual operations such as degassing or filtration stages, for example.
II-2. Spinning of the Solutions
On leaving the mixing and dissolving means, the solution is transferred in known manner towards a spinning block where it feeds a viscose pump. From this viscose pump, the solution is extruded through at least one spinneret, preceded by a filter. During its conveyance to the spinneret, the solution is gradually brought to the desired spinning temperature.
Each spinneret may comprise a variable number of extrusion capillaries, for example a single slot-shaped capillary for spinning a film, or in the case of a fibre several hundreds of capillaries, for example of cylindrical shape (diameter 50 to 80 micrometers, for example). From now on, the general case of spinning of a multifilament fibre will be considered.
On leaving the spinneret, therefore, a liquid extrudate of solution is obtained, formed of a variable number of elementary liquid veins. Preferably, the solutions are spun using the xe2x80x9cdry-jet-wet spinningxe2x80x9d technique using a non-coagulating fluid layer, generally air (xe2x80x9cair-gapxe2x80x9d), placed between the spinneret and the coagulating means. Each elementary liquid vein is stretched in this air-gap, by a factor generally of between 2 and 10 (spin-stretch factor), before penetrating into the coagulation zone, the thickness of the air-gap possibly varying to a great extent, according to the particular spinning conditions, for example from 10 mm to 100 mm.
After passing through the above non-coagulating layer, the stretched liquid veins penetrate into a coagulation device where they then come into contact with the coagulating agent. Under the action of the latter, they are transformed, by precipitation of the cellulose materials (cellulose or cellulose derivative) into solid filaments which thus form a fibre. The coagulation devices to be used are known devices, composed, for example, of baths, pipes and/or booths, containing the coagulating agent and in which the fibre being formed circulates. Preferably a coagulation bath located beneath the spinneret is used, at the exit from the non-coagulating layer. This bath is generally extended at its base by a vertical cylindrical tube, referred to as xe2x80x9cspinning tubexe2x80x9d, in which the coagulated fibre passes and the coagulating agent circulates.
xe2x80x9cCoagulating agentxe2x80x9d is understood to mean in known manner an agent liable to coagulate a solution, that is to say, an agent capable of rapidly precipitating the polymer in solution, in other words, of separating it rapidly from its solvent; the coagulating agent must be both a non-solvent of the polymer and a good solvent of the solvent of the polymer.
According to the invention, the coagulating agent used is an aqueous coagulating agent comprising at least one water-soluble additive, selected from the group consisting of ammonia, amines or the salts of these compounds, the additive being such that the pH of said coagulating agent is greater than 6.
Among the additives which correspond to the above definition, mention will be made, for example, of ammonia (aqueous ammonia), aliphatic or heterocyclic amines such as ethanolamine, diethanolamine, triethanolamine, ethylenediamine, diethylenetriamine, triethylamine, imidazole, 1-methyl imidazole, morpholine and piperazine, the preferred amines being primary or secondary amines comprising 1 to 5 carbon atoms.
Preferably, an organic or inorganic ammonium salt, and more preferably a salt selected from the group consisting of formates, acetates and phosphates of ammonium, mixed salts of these compounds or mixtures of these constituents, is used as additive, this ammonium salt possibly being, in particular, a salt of an acid present in the liquid-crystal solution, for example (NH4)2HPO4, (NH4)3HPO4, NaNH4HPO4, CH3COONH4 or HCOONH4.
Among those ammonium salts which are not suitable (pH of the coagulating agent not greater than 6), mention will be made in particular of (NH4)2SO4, (NH4)HSO4, (NH4)H2PO4 and NH4NO3.
The coagulating agent of the invention is preferably used on liquid-crystal solutions based on cellulose or cellulose formate dissolved in at least one phosphoric acid, such as described, for example, in applications WO85/05115 and WO96/09356 mentioned above: in this case, diammonium orthophosphate (NH4)2HPO4 is advantageously used.
The additive concentration of the coagulating agent (referred to as Ca) may vary to a great extent, for example from 2 to 25% (% total weight of coagulating agent), or even more, according to the particular conditions of implementation of the invention.
As far as the temperature of the coagulating agent (referred to as Tc hereafter) is concerned, it has been observed that low temperatures, in particular temperatures close to 0xc2x0 C., could in certain cases involve certain filaments sticking together during their formation (xe2x80x9cmarried filamentsxe2x80x9d). This upsets the spinning operations and is generally detrimental to the quality of the yarn obtained; thus, preferably, the coagulating agent of the invention is used at a temperature Tc greater than 10xc2x0 C., and more preferably close to ambient temperature (20xc2x0 C.) or above. It has been noted that the addition of a surfactant, for example isopropanol, or phosphate-based soaps, was another possible solution for eliminating, or at least reducing, the above difficulties.
According to the process of the invention, the amount of spinning solvent supplied by the solution in the coagulating agent is preferably kept at a level lower than 10%, and even more preferably lower than 5% (% total weight of coagulating agent), but in any case is controlled so that the pH of said coagulating agent is greater than 6, in accordance with the invention.
The total depth of coagulating agent through which the filaments pass during formation in the coagulation bath, measured from the entry to the bath to the entry to the spinning tube, may vary within a wide range, for example several millimeters to several centimeters. Nevertheless, it has been noted that an insufficient depth of coagulating agent might also involve the formation of xe2x80x9cmarried filamentsxe2x80x9d; thus, preferably, the depth of the coagulating agent is selected to be greater than 20 mm.
The person skilled in the art will be able to define the most appropriate coagulating agent according to the particular characteristics of the liquid-crystal solution to be coagulated, and he will be able to adapt parameters such as additive concentration, temperature or depth of coagulating agent to the particular conditions of implementation of the invention, in the light of the following description and examples of embodiment.
Preferably, the coagulating agent according to the invention is used in what is called the xe2x80x9cdry-jet-wet-spinningxe2x80x9d process, as described previously, but it could also be used in other spinning processes, for example what is called a xe2x80x9cwet-spinningxe2x80x9d process, that is to say, a spinning process in which the spinneret is immersed in the coagulating agent.
On leaving the coagulation means, the fibre is taken up onto a drive device, for example on motorised cylinders, to be washed in known manner, preferably with water, for example in baths or booths. After washing, the fibre is dried by any suitable means, for example by continuously passing over heating rollers preferably kept at a temperature of less than 200xc2x0 C.
In the case of a cellulose-derivative fibre, it is also possible to treat the washed, but not dried, fibre directly via regeneration baths, for example in an aqueous sodium hydroxide solution, in order to regenerate the cellulose and to arrive, after washing and drying, at a regenerated cellulose fibre.